1. Field of the Invention
The invention pertains to the field of miscible flooding for the secondary recovery of oil from subterranean reservoirs.
2. Brief Description of the Prior Art
Oil recovery by flooding with an extraneous fluid is a well known technique. One type of flooding utilizes fluids which are miscible with the oil in the reservoir. The fluids displace the oil in the reservoir toward the production wells. Miscible fluids also clean the reservoir oil from the pores of the sand and are, therefore, a more efficient flooding medium than water which is normally used.
If the miscible fluids are less dense than the reservoir fluids the efficiency of these miscible fluids is further enhanced by injecting them higher in the reservoir than the level where oil production is taken. This results in a vertical drive in the reservoir which takes advantage of natural density gradients and places the lighter fluid on top of the heaver fluid. Most miscible fluids are light hydrocarbons, solvents or gases for example, which are lighter than reservoir oil; therefore, a vertical drive is the most effective means of flooding the oil column with these miscible fluids.
The success of vertical flooding is dependent upon maintaining a well defined, discrete horizontal interface between the miscible fluid and the oil to be displaced. Mixing of the oil and the miscible fluid is detrimental to the flooding operation since the miscible fluid loses its ability to clean the oil in the reservoir as it becomes increasingly saturated with oil.
In every thick or steeply dipping bed a geothermal gradient exists with the temperature increasing with depth. This is called the geothermal gradient. Where there is adequate vertical permeability the geothermal gradient will cause convection mixing of the fluids at different levels in the reservoir. Thus, the hotter fluids low in the reservoir will tend to mix with the cooler fluids high in the reservoir as the reservoir attampts to gain equilibrium. Normally the reservoir temperature is much higher than the ambient temperature on the surface; therefore, if a miscible fluid at ambient surface temperature is injected into the top of a much hotter reservoir convection currents caused by the temperature and possible density differences will cause the hot oil to rise in the formation and mix with the cooler miscible fluid. The miscible fluid will thus be absorbed into the oil column and the miscible drive mechanism will be lost.
It is, therefore, an object of this invention to provide a method whereby a vertical miscible flooding operation may be carried on with a minimum of mixing of the oil and the reservoir fluid.
This may be accomplished by heating the injected fluid to a temperature higher than the reservoir temperature so that when the injected miscible fluid reaches reservoir depth it will be at a temperature higher than or equal to the reservoir temperature. When this is done the convection currents which normally rise from a hot fluid into an overlying cold fluid will no longer be able to rise since the hot miscible fluid is now above the cooler oil in the reservoir. By so minimizing the convection currents mixing will be reduced and the miscible fluid will remain intact as it drives the oil downward, This may be referred to as inverting the geothermal gradient.